Nylon 66, useful for making fibers, is currently made by batch or continuous processes wherein low molecular weight primary polyamide is converted to higher molecular weight secondary polyamide in a multi step polymerization process; such process is termed "amidation". A general amidation process is described in U.S. Pat. No. 3,763,113. High molecular weight and viscosity of the polyamide product are often desirable where strength and tenacity are important. See U.S. Pat. No. 3,551,548, column 1, lines 47-49. To achieve this result, an amidation catalyst, such as phenylphosphinic acid, 2-(2'-pyridyl)ethylphosphonic acid, or sodium hypophosphite, may be added to the starting salt strike, at a concentration of about 1 to 10 moles of amidation catalyst for every million grams of polymer (moles/10.sup.6 g). See U.S. Pat. No. 4,912,175.
The present invention teaches an alternate approach for increasing the molecular weight and viscosity of the product, involving adding a concentrate of the amidation catalyst(s) in nylon 66, hereafter called a "masterbatch", to the primary polyamide melt. Advantages of adding catalyst later in the process, rather than to the starting salt strike, include decreasing the transition time from one product to the next in a continuous process, decreasing expenses, since masterbatch addition can take the place of a separate solid phase polymerization step, improving spinning control since masterbatch addition can have a nearly immediate effect on threadline properties such as tension, and decreasing degradation.
European Patent Application No. 410,230 (Bayer AG), filed Jul. 13, 1990, describes a process for the manufacture of high molecular weight polyamide wherein at least one concentrate (masterbatch) is melt blended into catalyst-free polyamide and then the molecular weight of the blend is increased by solid phase polymerization. This application claims a wide range of thermoplastic matrices from polyamides to polyethylene and a wide range of catalysts. However, the amino and pyridyl phosphonic acids are not mentioned. Catalyst concentrations from 0.05 to 20 wt %, preferably 0.2 to 10 wt %, are claimed for the "masterbatches". Experimental examples are limited to matrices such as 6 nylon and polyethylene which do not generally show nylon 66's tendency towards gelation.
U.S. Pat. No. 3,551,548 discloses a process for the manufacture of fibers with increased relative viscosity by the addition of phosphorus compounds. The examples disclose that increased molecular weights are obtained with the simultaneous application of a vacuum and heat.
2-(2'-Pyridyl) ethylphosphonic acid (PEPA) and its diethyl ester (DPEP) have been claimed as amidation catalysts (U.S. Pat. No. 4,912,175), giving decreased thermal degradation in the presence of base (U.S. Pat. No. 5,116,919) , and as amidation catalysts preserving catalytic activity in the presence of alumina containing TiO.sub.2 (U.S. Pat. No. 5,142,000). U.S. Pat. No. 4,912,175, column 7, line 48 discloses PEPA's relatively low activity with respect to crosslinking during the amidation process. For example, under conditions where PEPA formed 2.5 moles of branches/10.sup.6 g of polymer (Example 8, U.S. Pat. No. 4,912,175) phenylphosphosphinic acid formed 4 moles of branches/10.sup.6 g of polymer (Comparative Example 1, U.S. Pat. No. 4,912,175).